Modulation measuring apparatus



Nov. 14, 19'44. H, `w KLINE 5 2,362,830

MODULATION MEASURING APPARATUS Filed Nov. 23, 1942 mmm Humm ing arectifier which means such as the diode II, having a cathode I2 PatentedNov. 14, 11944 UNITEIJv 'STATESv assaut MonULA'rroN MnvAsUamo. APPARATUSHalsey W. Kline, --Sch'enect'adm N. Y., assigner to General Electric NewYork Company, a co'r1 iol'ationv of Application November 23. -194:2,Serial No. 4466,613 4 claims. (ci. 17a-1715)' My invention relates tomeans for measuring or indicating therelationship between components ofa modulatedwave and more particularly to means for measuring orindicating the percentage or depth of modulation of radio fre- V quencyor carrier currents.

It is often desirable to measure or indicate the relationship betweenthe components of a modulated wave, especially during the adjustment andoperation of modulation equipment in communication systems.

ient to use at transmitting stations or at-points.`

of reception of transmitted signals where the operator usually does notpossess laboratory experience and where it is desiredto simplify theequipment as much as possible. 1

It is, therefore, an object of my provide new and improved means formeasuring vor indicating the relationship between components of amodulated wave. It is also an object of my invention to provide a newand improved modulation indicator. y

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. as to its organization andmanner of operation. together with further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawing in whichFig. 1 is a schematic vdiagram of one embodiment of my invention; Fig. 2is a schematic diagram of a second embodiment of my invention; and Figs.3 and 4 illustrate conditions of operation in the circuits of Figs.l.and 2. L

Referring to Fig. 1, there is illustrated a modulation indicating systemfor indicating the percentage or depth of a modulated wave obtained ifrom ,a source of modulated carrier voltage or signal source such asavariable-coupling radiofrequency transformer I0. The modulated signalvoltage is applied across a circuit comprismay be electron dischargeandan anode I3, and a loadresistance Il in series relationship. Thepositive enemations of the impressed voltage are rectified by the diodeFor example, it is desirable to operate a radio transmitter at a xedpercentage invention tov My invention itself, both` or rectier means to"pass currentfthrough the load resistance I4. During each ,such positivealternation the rectified voltage charges condenser I6 which isconnectedjacross the resistance Il. During the negative alternations ofthe impressedvoltage there is no passage of current through the diodeand therefore the condenser discharges through the resistance to ground.

This action of the condenser is shown by the u-heavy lines in Figs. 3`and .4. As is Well understood, the slope of the descending portion,rep-f resenting condenser discharge, depends upon the y time constant ofthe discharge circuit. Normally the time constant is chosen so that thel5 voltage curve is relatively smooth but the charg- .ing anddischarging of the condenser is exag gerated in the drawing to showclearly the operation of the circuit. In Fig. 3 there is illustrated anunmodulated carrier. Since the peak voltage of each alternation is thesame, the rectied voltage is a substantially constant unipotential wave.In Fig. 4 there is shown a condition existing with 100% modulation andacj cordingly the heavy line representing the voltage across theresistance I4 varies from zero to twice the maximum voltage of thecarrier wave.

From the foregoing it is understood that the voltage across theresistance I4 contains a D.-C.

component proportional to the carrier Wavey voltage and an A.C.component representing the audio or modulating voltage.

The audio voltage, or A.C. component, of the voltage appearing acrossthe resistance I4 is coupledl or impressed across a second circuit,comprising a second rectifier which may also be an electron dischargemeans such as a diode I8, having an anode I9 and a cathode 20, anda loadresistance 2I in series relationship. A coupling condenser 22 isinserted between the two cir-v 40 cuits to prevent the D.C component ofthe cara rier wave from being impressed across the second circuit. Inorder to prevent any radio frequency or carrier voltage from beingintroduced into the second circuit there is provided an inductance 36 inseries with capacitor 22 of such. value as to block radio frequencycurrents but to pass the audio component of ythe voltage introduced intothe second circuit.

The discharge means I8 passes current on the positive portions of the.audio voltage wave and at such times a unidirectional currentproportional to the audio` voltage passes through the 'load resistance2|. AIn order to provide a return path for the direct current in thesecond cir- 55 cuit there is provided a suitable circuit as, for

example, an iron core inductance 31 connected from' the anode I9 of thediode I8 to ground In order to smooth out the pulsating direct cur--rent in the resistor 2|, there is provided a capacitor 38, one side ofthe capacitor being connected to a point between the resistance 2| andthe cathode 20 and the other sideof the capacitor being connected toground.

Means is provided for comparing the rectified voltages in the twocircuits. In Fig. 1 this means takes the form. of a high resistancedirect current voltmeter as, for example, a voltmeter having aresistance of 20,000 ohms per volt connected between the cathodes I2 and20, i. e., across the positive ends of the resistances. I4 and 2l. Ifthe resistances I4 and 2| are properly chosen, the voltage dropsthereacross are equal` at 100% modulation` This is seen frominspectionof Figs. 3 and 4 in which the average D.-C. voltage across resistance I4is indicated by the heavy line in Fig. 3 and also by the dot-dash linel0 in Fig. 4. Under conditions of 100 percent modulation the averageD.C. voltage across the resistance 2l is also indicated by the dot-dashline 40. With the meter 23 connected as described above, the voltagestend to produce currents through the meter in bucking relationship and,under the conditions assumed, these oppos- -ing currents neutralize eachother, or cancel out.

For all conditions of less than 100 percent modulation the indicatingmeans or meter 23 reads up scale an amount proportional to themodulation percentage.

The amplitude of the meter readings for conditions of less than 100%modulation depend upon the voltage input to the rst circuit, that is,the voltage of the source I 0 of modulated signal. If an approximationof the depth of modulation is sufficient, a single instrument 23,connected as described, is suflicient but if a. relatively accuratereading is desired, means must be provided to insure that the inputvoltage is always of the same value. voltage to be adjusted, a suitableD.-C. ammeter I5 may be connected in the rst circuit. The ammeter I5measures the average rectied D..C. current in the iirst circuit, whichis proportional to the carrier current, and is not affected by the A.C.component of the rectiied voltage and,

therefore, is not affected by the depth or percentage of modulation. Thereading of meter I5 at the time of calibration of meter 23 may besuitablyV marked as ,represented by the Word set. Inorde'r to use theinstrument 23, the readingA of ammeter I5 is rst noted and then thevoltage of the input is adjusted, as by varying the coupling of thetransformer III, to establish a current flow of suilicient value tocause the needle of meter I5 to move to the-set position.

After this adjustment has been made, the other indicating device 23indicates accurately the depth of modulation. Y

The modulation indicator described herein may be used in connection.with a radio receiving device as Well as in connection with a radiotransmitter. If the device is to be used at a receiving station, theinput or source of modulation signal should be the output of the radiofrequency amplifier system, or the input to the second detector, if asuperheterody'ne type of receiver is employed inasmuch as the capacitivereactanceof a condenser varies with frequency and since the capacitor I6is in shunt with the resistance I4, the

.impedance of the circuit including Vcapacitor I 6 and resistanceY I4`varies as the modulation fre- To enable the inputV quency varies.Accordingly, the voltage across the resistance varies. If the value ofcapacitive reactance of capacitor I6 and the resistance of resistor I4are o1' the same order, the audio component across the diode 4variesover relatively wide limits, and the variation oi impedance o the diodecircuit according to the variations in the A.-C. component introduceserrors into' modulation indication. However, if the ratio of capacitivereactance to resistance is made relatively high as of the order of 3 to1, or greater, for example, the eiect will be relatively negligible.Thus for most accurate results it is desirable to choose the capacitorI6 and the resistor I 4- of such values that the ratio of capacitivereactance to resistance is suliiciently high tominimize the eiect ofvarying reactance of the capacitor with frequency.

In Fig. 2 there is illustrated a second embodiment of my invention whichis the same in operation as that shown in Fig. 1 but which employs adouble diode 24 containing vtwo electron discharge means in oneenvelope, such as a GHB type tube, instead of two separate diodes. Inthis embodiment of my invention there is utilized a single meter 25connected by means of a suitable double pole,A double throw switch 2B toread either the current through the resistance I4 or the voltage acrossthe cathodes I2p and 20'. In Fig. 2 like numerals are used for likeelements oi' Fig. 1.

The instrument 25 is connected to exible center contacts 2l and 28 whichare normally biased to engagement with another set of contacts 20 Vand30, respectively, which, in turn, are con- A Vnected to the cathode I2'through a suitable muitiplier or current limiting resistance 3i and tothe cathode 20', respectively. Thus in the normal position of the switchthe meter is connected as a voltmeter between the cathodes I 2' and.20'.The switch also comprises outer contacts 32 and 33 connected across aresistor 39, the resistor constituting a-shunt for the ammeter. Ii theoperating member or push button 34 is pressed inwardly to move theresilient arms 2l and 20 into engagement with the outer contacts 32 and33, the instrument 25'is connected as an ammeter across the shuntresistance 39.

The manner of operation is the same as that described for the embodimentshown in'Fig. 1.

While I have shown and described a parti- 'cular embodiment of myinvention, it wilibe obvious to those skilled in the art that changesand modications may be made without departing frommy invention in itsbroader aspects, and I, therefore, aim in the appended claims to coverall such changes and modifications as fall within the true spirit andscope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An apparatus for measuring the degree of modulation of a modulatedcarrier voltage com.- prising a source of modulated carrier waves havmgone terminal connected to a common point of predetermined fixedpotential, means including a rst load resistor having one end connectedto said common point for rectifying said carrier Wave to obtain aunidirectional voltage having an alternating component and a mean value,means including a second load resistor having one end connected 'to saidcommon point for rectiiying said alternating component of voltage toobtain a second unidirectional voltage, and a'dire'ct current indicatinginstrument having a. single coil connected to measure the di'erence ofsaid secaaeasso. ond unidirectional voltage and said meanfvalue of saidilrstunidirectional voltage.

if. An apparatus for measuring the degree of modulation oi a modulatedcarrier voltage com-f prising a source oi modulated carrier waves havingone terminal connected to a common point of predetermined fixedpotential, means including a ilrst load resistor having one endconnected to said common' point for' rectifying said. carrier waves toproduce a unidirectional voltage having an alternating component and amean v'alue.

means mcludlng a second load `resistor having one' end connected to saidcommon point for rectifying said alternating component of voltage to ob'tain a second unidirectional voltage. said second meansj'including asecond load' resistor having oneend connected to said common point forrectifying said lalternating component of voltage to obtain a secondunidirectionalvoltage, and a ,direct current indicating instrumenthaving a single coil connected to measure the diiierence of said meanunidirectional 'voltage and said second unidirectional voltage.

4. An apparatus for measuring '-Drising a source oi.' modulated carrierwaves havrectiiying means including means for preventing v the steadycomponent oi said first unidirectional voltage trom atecting said secondunidirectional voltage, and a direct current indicating instru. lmenthaving a'single coil connected between.

' 'foradiusting said .mean value to a predeter-L electrically spacedpoints on said load resistors to measure the diiierence or said meanunidirectional voli'fttgeL and said second unidirectional voltage. l

3. An apprtu for measuring the degree of modulation oi a modulatedcarrier voltage com- -prisinga' source oi modulated carrier waves havingone terminal connected to a common lpoint o! instrument having a singlecoil, means for coning one terminal connected to a common point ofpredetermined ilxed potential, means including a ilrstlload resistorhaving one end connected to said" commonpoint ior rectifying saidcarrier wave to'obtain a unidirectional voltage having an alternatingcomponent and a mean value, means mined desired intensity, meansincluding a sec'- ond load'resistor having one end connected to saidcommon point for rectifying said alternating component -of voltage toobtain a second uni directional voltage. a direct current indicatingnecting said coilbetween electrically spaced points on said resistors tomeasure the diilerence predetermined ilxed potentialr means including ailrst load resistor having one end connected tov said common point forrectitying said carrier wave to obtain a unidirectional voltage havingan alternating component and a mean value, means for adjusting said meanvalue oi' unidirectional voltage to a predetermined desired intensity,

or said mean unidirectional voltage and said secoudunidirectional'voltage. and switching means for temporarily rdisabling.said connection and connecting said coil to measure the intensity .of

said mean unidirectional voltage alone thereby to permit adjustment ofsaid mean voltage to saiddesiredintensity.

- Hanan! w. mmm

theV degree oi ,modulation of a modulated vcarrier voltage com-

